Question

List three types of physical evidence that indicate a chemical reaction has occurred.

Short answer

Three types of physical evidence that indicate a chemical reaction has occurred are: 1) Formation of a solid precipitate or a gas, such as the formation of barium sulfate when mixing barium chloride with sodium sulfate, or the production of carbon dioxide gas when mixing hydrochloric acid with sodium bicarbonate. 2) Change in color, like the yellow precipitate of lead(II) iodide produced when potassium iodide reacts with lead(II) nitrate. 3) Change in temperature due to exothermic or endothermic reactions, such as the heat released when magnesium burns in air or the cooling effect when ammonium chloride is mixed with water.

Step-by-step solution

1. Formation of a solid precipitate or a gas

When two solutions react to form a new, insoluble solid, it is called a precipitate. Precipitates are formed because the products of the reaction are no longer soluble in the solvent. This indicates that a chemical reaction has occurred. For example, mixing a solution of barium chloride (\(BaCl_2\)) with sodium sulfate (\(Na_2SO_4\)) results in the formation of a white precipitate, barium sulfate (\(BaSO_4\)). Also, the formation of a gas is an indicator of a chemical reaction. For instance, mixing an acid like hydrochloric acid (\(HCl\)) with a base like sodium bicarbonate (\(NaHCO_3\)) produces carbon dioxide gas (\(CO_2\)) that can be observed in the form of bubbles.

2. Change in color

A change in color is another indication that a chemical reaction has occurred. This change occurs because the reactants and products have different chemical structures, which affect the way they interact with light. For example, the reaction between potassium iodide (\(KI\)) and lead(II) nitrate (\(Pb(NO_3)_2\)) produces a yellow precipitate of lead(II) iodide (\(PbI_2\)).

3. Change in temperature (exothermic or endothermic reactions)

A chemical reaction may cause a change in the temperature of the system, which can be felt or measured. Reactions that release heat are called exothermic reactions, while those that absorb heat are called endothermic reactions. For example, when a piece of magnesium (\(Mg\)) is burned in air, it reacts with oxygen to form magnesium oxide (\(MgO\)), releasing a lot of heat in the process. This reaction is exothermic. On the other hand, mixing ammonium chloride (\(NH_4Cl\)) with water leads to an endothermic reaction, as it absorbs heat from the surroundings, making the mixture feel cold.

Key concepts

Formation of Precipitate

When two solutions interact and create a new solid that isn't soluble in the liquid, it's known as a precipitate. This is a tell-tale sign that a chemical reaction has occurred. The new solid forms because the products of the reaction aren’t willing to stay dissolved in the liquid like the original substances were. Precipitate formation is a fascinating process that clearly shows a chemical transformation.
For example, if you mix barium chloride ( BaCl_2 ) with sodium sulfate ( Na_2SO_4 ), they create a white solid called barium sulfate ( BaSO_4 ). This white substance, the precipitate, separates from the solution and settles out.
A similar indication of a reaction is the formation of a gas. Take the case where hydrochloric acid ( HCl ) reacts with sodium bicarbonate ( NaHCO_3 ). This reaction liberates carbon dioxide ( CO_2 ), visible as bubbles forming in the mixture.

• Precipitate: A sign of an insoluble product forming.
• Gas formation: Indicates reaction via bubble observation.

Color Change in Reactions

Color changes are another fascinating indicator that a chemical reaction has taken place. Often, when chemicals react, the resulting products have different attributes than the starting materials, including color. The molecular changes alter how substances absorb and reflect light, which is what we see as color.
One classic demonstration of this is the reaction between potassium iodide ( KI ) and lead(II) nitrate ( Pb(NO_3)_2 ). When these two come into contact, a bright yellow solid called lead(II) iodide ( PbI_2 ) forms, clearly visible as a vivid color change.

• Color change: Caused by different light absorption/reflection.
• Color change example: KI and Pb(NO_3)_2 mix to form yellow PbI_2.

These changes can make chemistry visually exciting and provide clear evidence that chemical processes are at play.

Temperature Change in Reactions

Temperature changes offer another tangible clue that a chemical reaction has occurred. These changes can be either exothermic, where the reaction emits heat, or endothermic, where it absorbs heat.
Exothermic reactions, like magnesium ( Mg ) burning in air to form magnesium oxide ( MgO ), release heat, often making the surroundings feel warmer. You might sense this as warmth on your skin during a lab experiment.
In contrast, endothermic reactions absorb heat. Mix ammonium chloride ( NH_4Cl ) with water, and the reaction cools the solution, causing it to feel unusually cold.

• Exothermic: Releases heat, warms surroundings.
• Endothermic: Absorbs heat, cools surroundings.

These temperature changes can be easily measured and often accompany the sound, light, or even the fizzing seen in reactions.